4 SHELL GALLERY. 
appearance in the form of an open pit, the mouth of which gradually 
closes up, leaving only a narrow slit in communication with the 
exterior. It is probable that in many forms the so-called ear is an 
organ by means of which the mollusc becomes acquainted with 
changes in the surface over which it is passing ; it is often found 
deeply imbedded in the substance of the foot, where it forms a 
closed vesicle. 
Sense of There is no doubt that the carnivorous Gastropoda are gifted 
smell. ^h a sense f smell, and throughout the series we observe patches 
of modified cells of the body-wall (the osphradium) which serve 
either as olfactory organs or as an apparatus for testing the nature 
of the water of respiration. 
The sexes The sexes are distinct in the most highly organized Mollusca, 
and repro k ufc are un ited in the same individual in some of the lower forms, 
such as Land-Snails, the Opisthooranchia (including the Bubble-Shells, 
Sea-Slugs, &c), and in some Bivalves. The reproduction of Mollusca 
is in all cases effected by means of eggs. In some instances the 
young are actually hatched within the oviduct of the parent, as in 
the Freshwater Snails (Vivipard) ; and apparently in most Bivalves 
the eggs are also retained within the valves until hatched. 
The ova of many molluscs are deposited in masses enclosed in 
capsules. Some of them are very wonderful and complicated 
structures. Those of the Cuttles and their allies are clustered 
like grapes, each capsule containing but a single embryo ; but in 
the Calamaries or Squids they form a radiating mass of elongated 
sacks, each containing from thirty to two hundred eggs, and it 
has been estimated that one of the spawn-clusters of the Common 
Squid (Loligo vulgaris) contains as many as 40,000 ova. Everybody 
knows the spawn-cases of the Common Whelk, found so abundantly 
on the sea-beach, consisting of a large number of yellowish capsules, 
heaped one upon another and forming an irregularly rounded mass. 
As many as five or six hundred capsules may be piled together in 
a single heap, each capsule containing several hundred eggs, of 
which perhaps only thirty or forty are hatched. 
In other genera, as Tethys, Doris, Eolis, &c, the eggs are 
contained in a spirally rolled ribbon or strap-like structure ; and 
some of the Natim build a somewhat similar capsule, composed of 
the eggs cemented together by sand and a gelatinous material, the 
whole forming two-thirds of a circle narrowed at the upper part. 
Terrestrial Molluscs deposit, in comparison with their marine 
relations, but very few eggs. They are sometimes covered by a 
